Color imaging devices, such as digital cameras, camera phones, and color copiers, use photodetector arrays to produce electronic signals that are capable of producing color images on a display or in a printer. A typical photodetector array has many individual photosites, each of which is responsive over a relatively wide range of wavelengths. For example, a photodetector produces an electrical signal whether it is illuminated with red, blue, green or infrared (IR) light. The magnitude of the electrical signal produced at different wavelengths of light varies according to the wavelength response of the photodetector. To form a color image, color pass filters are placed over individual photodetectors so that each photodetector is responsive to a relatively narrow range of light. For example, blue (pass) dye is placed over a photodetector in a detector array to obtain a blue detector. Similarly, red and green dye is placed on other photodetectors to obtain red and green detectors (generally “color detectors”). The diode array thus obtains spatial color data when imaging an object.
Unfortunately, the dyes used to obtain the color detectors do not filter out IR light very well. IR light illuminating the color detectors increases the noise level out of the color detectors. In other words, a color detector that is not being illuminated by light of its selected color will still produce an electrical signal if it is illuminated with IR light. IR illumination of color detectors can reduce the brightness of (“washout”) the colors and the contrast of the image because regions that appear dark to the observer's eye will appear lighter in the image if IR light is illuminating the photodetector.
Many techniques have been used to reduce the amount of IR illuminating color detector arrays. In digital imaging systems, such as digital still cameras (“DSCs”), video cameras, and camera-telephones (“camphones”), lids have been placed over the color detector array. The original function of the lid was to protect the sensor from dust particles. To save space in compact digital imaging devices, such as a camphone, IR filters have been used as a lid. As used herein, the terms “IR filter” and “IR-blocking filter” mean a filter that absorbs or reflects (filters out) IR light and generally transmits visible light.
One type of lid uses colored glass (“color glass”) that absorbs IR light (IR-blocking glass). Color glass is usually not used in camphones because of its thickness. Another type of lid uses an IR-blocking filter made of a series of layers of dielectric materials. Other lids use an IR-blocking filter on a colored glass substrate. In DSCs, IR-blocking and blur filters (also called an optical low-pass filter (“OLPF”)) are combined and are physically separated from the sensor and sensor lid
However, IR-blocking color glass lids are relatively thick (typically about 0.5 to 1.2 mm thick), which makes this approach undesirable for use in small, portable devices such as cell phones and digital cameras. IR-blocking color glass is also relatively expensive, and the amount of IR-blocking color glass depends on both the color density and thickness of the IR-blocking color glass.
Dielectric IR-blocking filters typically have 30–50 quarter-wave layers of dielectric materials coated on a plain glass substrate that is about 0.3 mm thick. The total dielectric stack height (i.e. all 30–50 layers) is typically about 3–5 microns. However, coating this many layers, typically in a vacuum deposition system, takes a long time and is therefore relatively expensive.
A stack this thick on such a thin substrate also can bend the substrate out-of-plane. Another problem arises from the wavelength shift with angle of incidence. In a typical dielectric IR-blocking filter the wavelength (e.g. cutoff wavelength) shifts 25 nano-meters (“nm”) with a 25-degree change in the angle of incidence from normal.
Wavelength shift can be reduced by using dielectric layers with higher refractive indices, but this generally requires more layers to be coated to achieve the same filter characteristic. Some wavelength shift can be corrected in the imaging device, but these techniques can be cumbersome and difficult to achieve, particularly in shallow (short light path length) optical assemblies, such as are found in camera phones and similar devices.
Furthermore, due to the high layer count and the thin substrate, dense, high-quality dielectric thin-film coating bends the substrate due to compressive stress and can cause yield loss in post coating processes, such as dicing the deposition substrate into filters. Because of the problems arising from dielectric thin-film IR-blocking filters, some users have abandoned this approach and have returned to using IR-blocking glass. However, color glass has issues with environmental stability and batch-to-batch variation.
Similarly, almost all the charge-coupled diode (“CCD”) and complementary metal-oxide-semiconductor (“CMOS”) image sensors have not only color dyes coated over the photodetectors, but also polymer microlenses. It is difficult to coat anything directly on top of the microlenses, especially when a high-temperature process is used. Therefore, it is desirable to provide an IR-blocking device for color detector arrays that avoids the problems mentioned above.